Fronthaul technologies, connecting remote radio units, RRU, or radio heads, RH, to base-band units, BBUs, have recently received a lot of attention in the telecom industry. The benefits of a C-RAN, centralized radio access network or cloud RAN, architecture can only be realized with fronthaul connecting distributed RRUs or RHs to centralized BBUs.
In fronthaul like the common public radio interface, CPRI, the radio signal is currently transported in the form of baseband IQ samples. In uplink at the RH, the received RF signal is down-converted and digitized in order to get baseband IQ samples. These samples are then processed further in the BBU. In downlink at the RH, the received baseband IQ samples from the fronthaul interface are converted to the desired RF signal by up-conversion and digital-to-analog conversion. The RF signal is then amplified and transmitted from the antenna.
Each IQ sample is quantized to a high number of bits, e.g. 20-30 bits in a floating- or fixed-point format, to keep the radio signal quality within standardized levels.
The current CPRI standard, version 6.0, specifies the fronthaul interface implementation. CPRI was originally designed as an internal interface between baseband and radio cards within a base station. It has now also come to been used in point-to-point fronthaul links between BBUs and RRUs for main-remote applications. CPRI specifies a multiplexing protocol to transmit the IQ samples of multiple antenna-carriers and the corresponding control/management messages together at a constant bit rate. It also provides a synchronization mechanism to synchronize the timing at the RRU side.
A major issue in current fronthaul implementations is the high bit rate requirements. For a single antenna, having for example a 20 MHz carrier bandwidth, 30.72 MHz sampling rate and 15+15 quantization bits per IQ sample, the required fronthaul capacity is about 922 Mbps although the traffic peak-rate is only 75 Mbps, for 64-QAM, quadrature amplitude modulation. Hence, a fronthaul link has to carry more than 12 times the actual air-interface traffic rate. Given a C-RAN architecture with a high number of RRUs connecting back to several BBUs, the high capacity demand for CPRI constitutes a scalability problem on the fronthaul links; they are developing bottlenecks.
To mitigate this, fronthaul compression schemes have been proposed utilizing inherent redundancy in the stream of IQ samples; the data rate can be reduced by lossy compression, trading radio signal quality for bit rate. Compression rates of 80% have been reported. The compressed fronthaul data is usually carried by a constant bit rate transmission like CPRI. Also, new Ethernet-based approaches have been proposed which carry the fronthaul data by Ethernet frames.
US 2014/0313914 A1 describes coordinated access and backhaul networks in which signalling parameters of the radio link are adjusted in response to degradations or improvements in the backhaul link, to try to maintain constant quality of the radio link as backhaul performance varies. It proposes decreasing the number of bits per sample for CPRI data transmitted over a microwave link that experiences degradation, or to change sample rate and/or resolution for a sampled analogue radio signal from a remote radio head.
WO2014167066 A1 discloses a mobile communication network and method for operating a mobile communication network in which backhaul capacity is determined and then radio capabilities of one or more base stations are adapted, preferably by the corresponding base station itself.